Vibration-based acoustic flowmeters with a vibration detector detecting vibrations caused by a standing wave

ABSTRACT

Vibration-based flowmeters are useable in inaccessible nuclear reactor spaces. Pipe-organ-type flowmeters include a passage with an opening constricted, and subsequent widening section. An extension and outlet that create turbulence in the flow at the outlet create a standing wave and vibration in the extension and/or entire flowmeter. A flow rate of the fluid through the flowmeter can be calculated using length of the passage and/or known properties of the fluid. Multiple flowmeters of customized physical properties and types are useable together.

RELATED APPLICATIONS

This application is a divisional of, and claims priority under 35 U.S.C.§§ 120 and 121 to, co-pending application Ser. No. 15/469,735 filed Mar.27, 2017, which is incorporated herein by reference in its entirety.

BACKGROUND

FIG. 1 is cutaway view of a related art reactor pressure vessel 1, suchas an ESBWR pressure vessel. Vessel 1 includes a core 15 within coreshroud 10. Core shroud 10 separates upward flow of coolant through core15 from downward flow in downcomer annulus 4, received from mainfeedwater line 3 and from liquid separated from steam separators anddryers. A feedwater cleanup system 12 may recirculate and filterfeedwater from reactor vessel 1 into main feedwater line 3, and one ormore control rod drives 50 may extend through a bottom of vessel 1 forcontrol and monitoring of the conditions in core 15. As liquid coolantboils in core 15, a heated mixture of steam and water flows upward intosteam separators and dryers that separate liquid water from thesteam-water mixture rising therethrough. Liquid coolant from the steamseparators and dryers is directed into downcomer annulus 4 between theouter wall 10 and inner wall of vessel 1 for recirculation. The driedsteam exiting steam dryers is then directed into main steam lines 2 forelectrical power production.

As described in ESBWR Design Control Document, Tier 2, 2013,incorporated by reference herein in its entirety, flow through core 15is derived from a heat balance calculation; there is no directmeasurement of fluid flow through core 15. The balance of heat andoverall energy created and extracted from core 15 allows operators toestimate or model whole core flow. For example, for an ESBWR at 100%power, 31,553 tons/hr of coolant may enter a lower core section. Thisderived core flow may be directly presented to operators as ameasurement of plant status as well as a diagnostic aid in transientconditions.

SUMMARY

Example embodiments include flowmeters that work through use of inducedvibration or sound in a particular flow, including those in inaccessiblenuclear reactor spaces. An example embodiment flowmeter includes a solidextension that blocks flow in a fluid flow path, such as in a nuclearreactor downcomer annulus between a core shroud and reactor vessel, anda detector that picks up vibrations caused by vortex shedding around theextension. A computer processor can use the detected frequency of thevibrations to calculate a flow rate of the fluid past the extension,such as by using known shape of the extension, fluid flowcharacteristics such as density, an empirical relationship betweenvibration frequency and flow speed, etc. For example, the computerprocessor may use a Strouhal number for an extension having a roundouter surface to calculate a flow speed from the surface diameter anddetected vortex shedding frequency. The extension may have a naturaloscillation frequency to match the frequency of vortex shedding atexpected flow speed(s) in order to permit direct monitoring of vibrationin the extension. As such, several extensions, each with uniqueoscillation frequencies, may be used to cover a range of frequencies ofvortex shedding and thus a complete range of flow speeds in the space.

Another example embodiment may be a pipe-organ-type flowmeter where thefluid flows through a passage in the flowmeter. An opening of theflowmeter may be constricted, followed by a widening section with awedge or other extension and outlet that create turbulence in the flowat the outlet. The turbulence or vorticing in the passage create astanding wave in the fluid flow, which induces vibration in theextension and/or entire flowmeter. A vibration detector and processorcan transform the vibration frequency into a flow rate of the fluidthrough the flowmeter using the length of the passage and/or knownproperties of the fluid. The passage may widen at an exit and/or includeseveral bends or curves to achieve a desired length and frequency ofvibration in the passage. Embodiments are useable together, such asinstalling a pipe-organ-type flowmeter through an extension-typeflowmeter with vibrational detection of both vortex shedding andstanding wave oscillations detected in both. Of course, embodiments areuseable separately, with independent or exclusive installations in theflow passage.

Example methods include installing an extension, potentially with apipe-organ-type opening into a passage where flow is desired to bedirectly measured and then detecting a frequency of vibration caused bya standing wave or vortex shedding in/around the extension. Flow rate isthen calculated based on the detected oscillation frequency. Forexample, using a guessed Strouhal number, a width of the extension, andthe frequency, a flow rate can be determined. This flow rate may then bechecked by calculating a Reynolds number of the flow using thecalculated flow rate and comparing an empirical Strouhal number fromthat Reynolds number against the guessed Strouhal number. If theempirical and guessed Strouhal numbers are sufficiently close, the rateis accepted; otherwise, a new Strouhal number is guessed and the methodrepeated. Similarly, example methods can compare flow rates determinedwith a guessed Strouhal number in extension-type embodiment(s) with aflow rate determined by pip-organ-type embodiment(s) to verify accuracyof determined flow rate. Still further, a lack of vibration detected ina pipe-organ-type embodiment may indicate the presence of two-phaseflow.

BRIEF DESCRIPTIONS OF THE DRAWINGS

Example embodiments will become more apparent by describing, in detail,the attached drawings, wherein like elements are represented by likereference numerals, which are given by way of illustration only and thusdo not limit the terms which they depict.

FIG. 1 is an illustration of a related art nuclear power reactor heatbalance schematic.

FIG. 2 is perspective view of an example embodiment acoustic flow meter.

FIG. 3 is an illustration of vortex shedding and induced vibration in anexample embodiment acoustic flow meter.

FIG. 4 is an illustration of an empirical relationship between Reynold'sand Strouhal numbers.

FIG. 5 is a profile section of another example embodiment acoustic flowmeter.

FIG. 6 is an illustration of an example embodiment flow measurementsystem using several acoustic flow meters.

FIG. 7 is a graph of an example empirical relationship between Strouhaland Reynolds numbers for a fluid flow.

DETAILED DESCRIPTION

Because this is a patent document, general broad rules of constructionshould be applied when reading and understanding it. Everythingdescribed and shown in this document is an example of subject matterfalling within the scope of the appended claims. Any specific structuraland functional details disclosed herein are merely for purposes ofdescribing how to make and use example embodiments or methods. Severaldifferent embodiments not specifically disclosed herein fall within theclaim scope; as such, the claims may be embodied in many alternate formsand should not be construed as limited to only example embodiments setforth herein.

It will be understood that, although the terms first, second, etc. maybe used herein to describe various elements, these elements should notbe limited by these terms. These terms are only used to distinguish oneelement from another. For example, a first element could be termed asecond element, and, similarly, a second element could be termed a firstelement, without departing from the scope of example embodiments. Asused herein, the term “and/or” includes any and all combinations of oneor more of the associated listed items.

It will be understood that when an element is referred to as being“connected,” “coupled,” “mated,” “attached,” or “fixed” to anotherelement, it can be directly connected or coupled to the other element orintervening elements may be present. In contrast, when an element isreferred to as being “directly connected” or “directly coupled” toanother element, there are no intervening elements present. Other wordsused to describe the relationship between elements should be interpretedin a like fashion (e.g., “between” versus “directly between”, “adjacent”versus “directly adjacent”, etc.). Similarly, a term such as“communicatively connected” includes all variations of informationexchange routes between two devices, including intermediary devices,networks, etc., connected wirelessly or not.

As used herein, the singular forms “a,” “an,” and “the” are intended toinclude both the singular and plural forms, unless the languageexplicitly indicates otherwise with words like “only,” “single,” and/or“one.” It will be further understood that the terms “comprises”,“comprising,”, “includes” and/or “including”, when used herein, specifythe presence of stated features, steps, operations, elements, ideas,and/or components, but do not themselves preclude the presence oraddition of one or more other features, steps, operations, elements,components, ideas, and/or groups thereof.

It should also be noted that the structures and operations discussedbelow may occur out of the order described and/or noted in the figures.For example, two operations and/or figures shown in succession may infact be executed concurrently or may sometimes be executed in thereverse order, depending upon the functionality/acts involved.Similarly, individual operations within example methods described belowmay be executed repetitively, individually or sequentially, so as toprovide looping or other series of operations aside from the singleoperations described below. It should be presumed that any embodimenthaving features and functionality described below, in any workablecombination, falls within the scope of example embodiments.

The Inventors have newly recognized that a heat-balance derivation ofreactor coolant/moderator flow gives only general, non-precise valuesfor whole core flow. Direct and/or independent measurement of fluid flowat exact positions within a nuclear reactor, especially in a larger coreand reactor like an ESBWR, provides an important verification to heatbalance flow estimates as well as more precise reflection of reactorconditions and transient scenarios. Particularly, accurate measurementof total flow down into a downcomer annulus, accounting for allrecirculated and injected feedwater, is useful for confirming totalcoolant/moderator flow up through a core and precise, direct flowmeasurements enable better calculation of stability margins required forcore safety compliance as well as better strategic operation of controlelements. Direct/non-derived measurement of core flow is further helpfulat startup and shutdown and to diagnose undesired or transientscenarios, including core power oscillations and burnup variations.Example embodiments described below address these and other problemsrecognized by the Inventors with unique solutions enabled by exampleembodiments.

The present invention is acoustic flowmeters and methods of using thesame. In contrast to the present invention, the small number of exampleembodiments and example methods discussed below illustrate just a subsetof the variety of different configurations that can be used as and/or inconnection with the present invention.

FIG. 2 is an illustration of an example embodiment acoustical flowmeter100 useable in an operating nuclear reactor environment. As shown inFIG. 2 , example flowmeter 100 includes projection 101 shaped to extendperpendicularly to and into a fluid flow 20. Projection 101 may besecured to a base to ensure its continued perpendicular positioning evenin heavy flows. For example, projection 101 may be installed on a wallof a downcomer, such as an exterior of core shroud 10, of a boilingwater reactor or ESBWR (FIG. 1 ) and, during operation, immersed inpurified water or other coolant/moderator 20 flowing past projection101. Because of the size of projection 101 and typical flow velocity,flow 20 undergoes diversions 21 from laminar flow around projection 101,creating a small amount of turbulence and voiding around projection 101.

FIG. 3 is a front view of example embodiment flowmeter 100 in fluid flow20. As seen, at sufficient velocity, flow 20 breaks, or sheds, fromprojection 101, creating a trailing void 25. As flow diversions 21proceed downstream in surrounding flow, they generally correct back froman initial diversion 21, oscillating back and forth. This createsopposite voids, or turbulent areas, 25 in opposite positions as eachdiversion 21 corrects back to vertical flow downstream. This phenomenon,known as vortex shedding, thus creates an oscillating flowback-and-forth between voids 25, of frequency f (showing one voidingcycle in a given time period). The vortex shedding may induce vibrationin projection 101 and nearby structures of frequency f, or relatedvibration oscillation.

Oscillations in position and pressure—vibration—in projection 101 may bedetected and/or converted to electrical signals by vibration detector90, such as a microphone, another acoustic pickup, transducer, etc.Vibration detector 90 may be directly coupled with projection 101, witha pickup embedded in or connected to projection 101. Alternatively,vibration detector 90 may be downstream or even embedded in or onanother side of a structure such as core shroud 10 from projection 101yet still pick up vibration caused in projection 101 or other nearbystructures by the vortex shedding. Vibration detector 90 may beself-powered by the vibration, and transmit a relatively smallelectrical signal that may be amplified elsewhere; additionally oralternatively, vibration detector 90 may include a battery or electricalgrid connection and transmit fully amplified electrical signals and/orwireless signals generated from the vibration. For example, a calibratedacoustical pickup on an external side of reactor pressure vessel 1 maybe powered through electrical connections in a containment building andmay detect the vibration in fluid flow created by projection 101 in anannular downcomer inside reactor vessel 1 when properly tuned toexpected frequencies and filtering out other background noise fromvessel 1 and surroundings.

With a vibration frequency detected by vibration detector 90, a flowrate may be determined. This determination may be made by computerprocessor 95 receiving vibrational frequency from vibration detector 90,including a local processor and/or a remote plant computer receiving,and potentially amplifying and cleaning, the data through acommunicative connection with vibration detector 90. The determinationof flow rate may be made from a determinative or historically-measuredrelationship between the vortex shedding frequency f and flow velocity.For example, if projection 101 is substantially cylindrical with agenerally smooth, wetable surface, the relationship between vortexshedding frequency from projection 101 and rate of fluid flow 20 isgiven by the Strouhal equation:St=(f*d)/U  (1)where f is the detected frequency by vibration detector 90 or a vortexshedding frequency derived from the same; d is the diameter ofprojection 101 into flow 20, and U is the steady stream velocity of flow20 being solved for. The Strouhal number, St, may initially be unknownbut guessed at using historically-accurate numbers for flow rates or areasonable guess, such as 0.2, for typical downcomer flow rates andconditions.

The Strouhal number can be verified through empirical data, yielding aneven better result. FIG. 4 is an example graph relating Reynolds andStrouhal numbers from an empirical determination in controlledconditions. As such, by calculating the Reynolds number for thedetermined flow rate, the Strouhal number can be checked by the graph ofFIG. 4 to ensure a sufficiently close fit. Reynolds number, assumingopen flow around projection 101 in large hydraulic diameter conditions,such as in a typical boiling water reactor downcomer, is calculated by:Re=(ρ*U*d)/μ  (2)where p is fluid density, U is the determined flow rate from equation(1), d is the diameter of extension 101, and μ is the fluid viscosity.The fluid density and viscosity are known from the coolant/moderatortype and temperature and pressure in the downcomer or other flow spacethat surrounds projection 101. The calculated Reynolds number, Re, isthen used to get an associated Strouhal number from FIG. 4 , and thismay be compared to the guessed or otherwise originally-used Strouhalnumber St in equation (1) to ensure a match. If the originally-usedStrouhal number is outside of a tolerance or otherwise does notsufficiently match the associated Strouhal number from FIG. 4 , a newStrouhal number may be guessed. The new guessed Strouhal number willlikely be larger, but the results of the comparison will indicate whichdirection the guess should move. Once there is agreement betweenempirically-associated and guessed Strouhal numbers for a particularflow rate, the calculated flow rate is verified.

Of course, other analyses may be used to calculate flow rates fromdetected acoustic vibration information. For example, a direct empiricalrelationship between flow rate and vortex shedding frequency (or anyshape/characteristic of extension 101) may be known, and the detectedfrequency may be used to directly calculate flow rate. Such analyses maybe performed on an appropriately-programmed or configured computerprocessor(s) using the known physical characteristics of extension 101,fluid properties, and flow path characteristics. The computer may outputthe calculated flow rate from each example embodiment acousticalflowmeter 100 to a plant computer and/or operator, such as in a controlroom, for direct knowledge of coolant flow rate.

As discussed above, vortex shedding frequency f may be directly measuredby vibration detector 90 and/or may be measured by vibration inextension 101, either directly or through a known relationship betweenvortex shedding frequency and extension vibration. If direct measurementof frequency in extension 101 is desired, such as for simplification oreasier vibration detection in a solid structure like extension 101,extension 101 may be configured to ensure a vibrational characteristicthat matches, or may be determinatively calculated from, vortex sheddingfrequency f used to determine flow velocity.

Extension 101 may be configured with a natural frequency coveringexpected vortex shedding frequencies, so that extension 101 willresonate, and vibrate at, the vortex shedding frequency f Most solidstructures will vibrate only when exposed to certain frequencies ofvortex shedding, so appropriate configurations and numbers of exampleembodiment acoustical flowmeter(s) 100 may be used in a flow 20. Forexample, the typical bandwidth of natural frequencies that will causeextension 101 to vibrate at a vortex shedding frequency f is given by:ω_(BW)=ω_(n)√((1−2ζ²)+(ζ⁴−4ζ²+2))  (3)where ω_(n) is the natural frequency of extension 101, and ζ is thedamping ratio of extension 101 in the fluid determined from a modalanalysis. A desired damping ratio may be achieved by varying stiffnessof any vibrating part in a length/axial or width/radial direction, forexample, to cover expected resonance frequencies for a given flow.Additive manufacturing, building a part, such as extension 101, throughsequential add-ons or welds may permit easy customization of dampingratio and thus bandwidth of natural frequencies covered.

For example, if extension 101 is a cylindrical extension fabricated of arigid material such as a metal with a relatively constant modulus ofelasticity over a given range (i.e., does not plastically deform atexpected frequencies), then the natural frequency ω_(n) is given by:ω_(n)=α²√(E*I)/(m*L ⁴))  (4)where E is the modulus of elasticity; I is the moment of inertia (πr⁴/4for a cylinder); m is the mass; and L is the length of extension 101.For a structure having more than one harmonic node or natural frequency,a, is the modal number, which may be empirically determined. Usingcustomization, such as through additive manufacturing, specific radii,lengths, and/or stiffnesses may be achieved to give extension 101desired natural frequencies and modes, based on expected flow conditionsand/or desired frequency coverage. The natural frequency is then dividedby 2π to correlate with vortex shedding/vibrational frequency f used inequations (1) and (2).

As seen in the forgoing equations (3) and (4), extension 101 may be of amaterial, length, and radius that will give good coverage of expectedvortex shedding frequencies f That is, example embodiment acousticflowmeter 100 will vibrate and produce useable data for a flow raterange in flow 20 when properties of extension 101 are configured inaccordance with the forgoing parameters to ensure resonance with thevortex shedding at expected flow rates. Of course, other shapes andstructures of extension 101 may be chosen, with known materialproperties and natural frequencies determinable, to be useable asstructures that vibrate in accordance with fluid flow.

In the instance that extension 101 has a natural frequency bandwidththat covers only a subset of potential flow rates of interest, multipleextensions 101 may be used, and/or multiple example embodimentacoustical flowmeters 100 may be implemented, with varying naturalfrequencies to cover a flow rate range of interest. For example, asshown in FIG. 6 , multiple example embodiment flowmeters 100 havingdiffering extension characteristics may be mounted on core shroud 10 ina downcomer space to measure flow rates from startup, to steady-stateoperations, to transient conditions. Example embodiment flowmeters 100may each include their own acoustical pickup specifically tuned to theirunique resonance bandwidth or may share vibration detectors andcommunicative connectivity among each other.

As shown in FIG. 2 , example embodiment flowmeter 100 may includeanother example embodiment flowmeter 200. Alternatively, exampleembodiment flowmeter 200 may be free-standing, without the operation offlowmeter 100. As shown in FIG. 5 , example embodiment flowmeter 200includes an internal passage 204 that allows fluid flow 20, such asliquid coolant in a downcomer annulus, to pass through flowmeter 200.Internal passage 204 may be inside extension 101, for example.

Example embodiment flowmeter 200 may include a top opening 201 thatallows fluid flow 20 to enter flowmeter 200. Example embodimentflowmeter 200 may expand, much like a pipe organ design, in direction offlow 20, to a wedge 203 interrupting flow 20 at opening 202. Thecombination of expansion following top opening 201, wedge 203, andopening 202 causes flow 20 to vortex and be partially expelled throughopening 202. The remainder of the flow may pass down and out of exampleembodiment flowmeter 200 via passage 204.

The vorticing inside example embodiment flowmeter 200 creates a standingwave in flowmeter 200, forcing vibration of flowmeter 200 at a frequencydependent on velocity of fluid flow 20. As shown in FIG. 7 , thevelocity of the fluid alone may be determined from a linear relationship701 between flow velocity and frequency produced in wedge 203 usedalone. Alternatively, as shown in FIG. 7 , the frequency produced inexample embodiment 200 including internal passage 204 like a pipe willjump between harmonic levels 702 based on flow velocity. Therelationship between different frequencies and flow speed ranges isgiven by:f _(n) =n*ν/2L  (5)where f_(n) is the vibrational frequency, ν is the speed of sound in thefluid, L is the length of passage 240, and n is the node, or number ofthe harmonic, associated with fluid flow speed through the pipe.

A vibration detector 90 can detect vibration in example embodimentflowmeter 200 through the same type of placement and/or calibration aswith example embodiment flowmeter 100 for the frequencies expected ofexample embodiment flowmeter 200. Knowing the length of passage 240, thespeed of sound in the flowing coolant from its type, temperature, andpressure; and the detected vibrational or oscillating frequency ofexample embodiment flowmeter 200, the harmonic number n can bedetermined using a local or remote processor. As seen in FIG. 7 , eachharmonic level 702 is associated with a distinct range of flowvelocities into example embodiment flowmeter 200, which may bedetermined empirically through observation of the harmonic range or bynodal analysis of flowmeter harmonics. By determining the frequency,f_(n), the matching flow rate range can be established and thusindependently measured in a nuclear reactor.

As shown in FIG. 5 example embodiment acoustic flowmeter 200 may includean expanding or flared opening 205 for passage 204. The increased areaof opening 205 may better conduct sound waves generated by resonance inexample embodiment acoustic flowmeter 200 into surrounding media forpickup by vibration detector 90. Passage 204 may also be S-shaped orcurved to lengthen its flow path, thus lowering the frequency f_(n) asshown by equation (5). A lower frequency may be more easily detectableor discriminated from ambient noise and vibration by detector 90.Similarly, passage 204 may be reconfigured into any longer or shortershape to suit the needs of vibration detection while ensuring fluidthrough-flow. For example, passage 204 may be completely straight andlack any flared opening 205 for manufacturing simplicity, while stillfunctioning as an acoustic flowmeter.

All elements of example embodiment flowmeter 200 may be additivelymanufactured into extension 101 of example embodiment flowmeter 100through machining, molding, welding, etc. components of each exampletogether as shown in FIG. 5 . Any number of example embodimentacoustical flowmeters 200 may be used in a single extension, or frommultiple extensions. Different acoustical flowmeters 200 may includedifferent lengths, wedge shapes, or other characteristics to coverdifferent flow rates with frequency nodes and/or produce differentfrequencies for detection and verification.

When used together, example embodiment flowmeters 100 and 200 may beused to verify each other and ensure a fluid velocity reading from onefalls within an acceptable range of another. Additionally, becauseexample embodiment flowmeter 200 will not resonate when subjected toradically-different densities in flow 20, such as two-phase flowcontaining steam, flowing into opening 201 while example embodimentflowmeter 100 will vibrate in 2-phase flow, a working/non-workingdisparity between example embodiment flowmeters 100 and 200 using a sameextension may indicate the presence of two-phase flow at a particularlocation. Such two-phase flow may alert operators to an undesirablecondition, such as steam dryer faults, feedwater overheating, shroudleakage, etc. Alternatively, each example embodiment acoustic flowmeter200 may be its own structure independently mounted in a flow 20 ofinterest, without use of any example embodiment acoustical flowmeter100.

Example embodiments and methods thus being described, it will beappreciated by one skilled in the art that example embodiments may bevaried and substituted through routine experimentation while stillfalling within the scope of the following claims. For example, althoughexample embodiments are discussed in usage with purified water flowingin an annular downcomer, a variety of different coolant types and flowpassages are compatible with example embodiments and methods simplythrough proper dimensioning and material/length selection of exampleembodiments—and fall within the scope of the claims. Such variations arenot to be regarded as departure from the scope of these claims.

What is claimed is:
 1. An acoustic flowmeter for use in a downcomerannulus of a nuclear reactor, the acoustic flowmeter comprising: a solidmaterial defining a passage with a top opening and exit through which afluid can flow, wherein the top opening has a smaller area than thepassage; an extension extending into the passage positioned with a sideopening in the passage, wherein the extension and the side opening areconfigured to cause vorticing in the flow and to cause fluid to flowthrough the side opening, so as to create a standing wave in thepassage; and a vibration detector paired with the solid material,wherein the vibration detector is configured to detect an oscillation inthe flowmeter caused by the standing wave and report a frequency of theoscillation.
 2. The acoustic flowmeter of claim 1, wherein the passagewidens at the exit.
 3. The acoustic flowmeter of claim 1, wherein thepassage includes two 90-degree bends.
 4. The acoustic flowmeter of claim1, further including: a computer processor coupled with the vibrationdetector, wherein the computer processor is configured to calculate aflow rate of the fluid from the frequency, based on a length of thepassage and a harmonic number of the oscillation.
 5. The acousticflowmeter of claim 1, wherein the extension and the side opening are ata same level in a direction of flow in the passage so that the extensionand vorticing cause fluid to flow out of the side opening.
 6. Theacoustic flowmeter of claim 5, wherein the passage includes a wideningsection past the top opening of the passage in the direction of flow,and wherein the same level of the extension and the side opening is pastthe widening section in the direction of flow.
 7. The acoustic flowmeterof claim 6, wherein the widening section is frustoconical.
 8. Theacoustic flowmeter of claim 7, wherein the vibration detector is anacoustical pickup outside the downcomer annulus.
 9. The acousticflowmeter of claim 5, wherein a space in the passage defined between theextension and the side opening at the same level is larger than the topopening of the passage.
 10. The acoustic flowmeter of claim 1, whereinthe solid material is configured to be mounted in the downcomer annulusso as to extend into reactor coolant flow in the downcomer annulus. 11.An acoustic flowmeter system in a nuclear reactor, the systemcomprising: a core shroud surrounding a core of the nuclear reactor; areactor pressure vessel, wherein an outer surface of the core shroud andan inner surface of the reactor pressure vessel form a downcomerannulus; a solid material defining a passage with a top opening and exitthrough which a fluid can flow, wherein the top opening has a smallerarea than the passage, wherein the solid material is in the nuclearreactor downcomer annulus secured to the core shroud and extendstransversely into the downcomer annulus so as to direct fluid flow fromthe downcomer annulus into the passage; an extension extending into thepassage positioned with a side opening in the passage, wherein theextension and the side opening are configured to cause vorticing in theflow and to cause fluid to flow through the side opening, so as tocreate a standing wave in the passage; and a vibration detector pairedwith the solid material, wherein the vibration detector is configured todetect an oscillation in the flowmeter caused by the standing wave andreport a frequency of the oscillation.
 12. The acoustic flowmeter ofclaim 1, further including: a computer processor coupled with thevibration detector and configured to calculate a flow rate of the fluidfrom the frequency, wherein the computer processor and the vibrationdetector are remote from the solid material.